Modern aircraft are thermally and acoustically insulated in order to ensure a thermal comfort and to limit the noise level in the cabin. Substantially the noises to be reduced in this context are usually created outside the aircraft, for example by the turbulent flow boundary layer on the fuselage and by the exhaust jet of the engines. In addition to increasing comfort demands by passengers, higher cruising speeds, as well as the acoustically disadvantageous characteristics of aircraft fuselages of fiber composite materials in direct comparison with monolithic metallic materials, represent an increasing challenge with respect to the acoustic insulation. Particularly the acoustic efficiency requirements of the insulation with respect to sound damping and sound absorption continuously increase for this reason and cannot be met with conventional insulation structures.
The thermal insulation of an aircraft fuselage—the so-called primary insulation—is usually composed of mat-shaped insulation assemblies that consist of glass wool of relatively low density (for example less than 10 kg/m3) in a thin foil wrapping. Mats for a skinplate region between the frames above the stringers used as longitudinal reinforcement of a fuselage, as well as mats for wrapping the frames, are used in this case.
This conventional thermal insulation also fulfills the function of the acoustic insulation and therefore contributes to ensuring the cabin comfort with respect to temperature and noise. The acoustic efficiency of conventionally used glass wool assemblies is relatively high in the high-frequency range that essentially includes the speech intelligibility range, whereas only an insignificant sound damping takes place at frequencies below 500 Hz. If stricter demands with respect to the noise comfort in the interior need to be met as it is the case, for example, in corporate or private aircraft, it is common practice to use, among other things, needled felt materials that are fixed, for example, above the frame heads, to some extent also such that they face away from the fuselage.
According to publication WO 2005/095206, as well as publication WO 2006/114332, multilayer insulation assemblies that are composed of different materials can be used for increasing the acoustic comfort within the cabin of an aircraft. A person skilled in the art is furthermore familiar with the fact that the degree of sound damping of a structure can be increased, particularly in the low-frequency range (for example lower than 500 Hz), by increasing the mass of the outer skin of the aircraft or by using an additional layer with relatively high specific density in the insulation structure. In this case, the increase in the weight per unit area of the aircraft fuselage can be realized, for example, in the form of a direct application of so-called damping coverings or heavy layers onto the skinplates.
In addition, the advantageous acoustic properties of double-wall structures in comparison with single walls are also known. At an altogether identical weight of the measure for reducing the sound transmission, higher degrees of sound damping are achieved with a distribution over two wall elements than with the concentration of the same weight on a single wall. The weight available for acoustic measures is limited, particularly in aircraft construction, such that a person skilled in the art would prefer the construction of double-wall or multi-wall structures in order to provide the best noise reduction possible.
In aforementioned WO 2006/114332, an insulation structure for use in corporate aircraft is disclosed, in which a double-wall structure consisting of outer skin and a heavy foil is produced by utilizing a combined assembly of a porous absorber and a heavy foil in connection with the outer skin of an aircraft. However, this structure has different disadvantages. At the so-called double-wall resonant frequency that essentially results from the mass of the heavy foil and the distance between the heavy foil and the outer skin of the aircraft, the double wall produced by the structure causes a drop in the degree of sound damping and only exhibits the known advantageous acoustic properties above this double-wall resonant frequency. In addition, the fuselage, the heavy foil and the interior lining panels result in a multi-wall structure with other resonant effects and associated drops in the degree of sound damping, in particular, in the sidewall region of aircraft. These double-wall and multi-wall resonances are created due to the fact that the air enclosed between the walls in the regions of reinforcing components acts as a spring such that an oscillatory spring-mass system is created.
The heavy foil used also acoustically covers the primary insulation arranged on the aircraft fuselage. However, this means that the sound energy between the heavy foil and the cabin lining can no longer be absorbed such that the effective absorber layer thickness on the cabin side is reduced.
In addition, a closed heavy foil within a thermal insulation structure acts as a vapor barrier such that the utilization of the heavy foil results in the formation of condensation water that is absorbed by the adjacent porous insulation material. Since the insulation material is completely covered, it is difficult or entirely impossible to dry this material such that the weight of the materials in the thermal and acoustical insulation continuously increases over the service life of the aircraft.